The present invention relates to an apparatus for, and a method of improving the fuel economy of internal combustion engines used in hybrid vehicles and used in stationary applications.
When generating power or providing torque with an internal combustion engine for the purpose of propelling a vehicle, or for providing power for stationary applications, it is desirable to maximize energy efficiency and minimize net fuel usage.
It is well-known that the energy efficiency of an internal combustion engine is dependent on matching the engine characteristics to the intended application. Unfortunately, only a limited number of engine sizes are available which makes it difficult to choose an engine with optimal characteristics, even in the case of steady power requirements. Furthermore, many engines are employed in applications with varying loads for which the ideal engine characteristics also vary. Therefore, it is usually the case that no single conventional engine is the best choice for the full set of operating conditions to be encountered.
Known in the art for purposes of improved fuel economies are numerous “variable displacement engine” designs and strategies using “cylinder deactivation,” i.e. operating with a reduced number of active cylinders, as conditions allow. Long period, commercially available deactivation strategies in automobiles with only a subset of the cylinders capable of deactivation operate at somewhat improved fuel efficiencies but do not operate at substantially maximum fuel efficiency over most of the operating conditions to be experienced. This is because more than the minimum, fractional number of cylinders necessary to achieve the desired output at peak fuel efficiency are employed most of the time, and therefore, the active cylinders must still be purposefully de-rated or throttled to ensure the desired output.
Another known approach is often referred to as “skip fire” engine control. Skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, for example, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. This is contrasted with conventional variable displacement engine operation in which a fixed set of the cylinders are deactivated during certain low-load operating conditions. In general, skip fire engine control is understood to offer a number of potential advantages, including the potential of significantly improved fuel economy in many applications. Such skip fire control was described, for example by Förster (U.S. Pat. No. 4,509,488) and in a number of co-assigned patents such as Tripathi (U.S. Pat. No. 7,954,474). During skip fire operation, a cylinder may be deactivated during skipped cycles so that air is not pumped through the cylinder, which helps reduce pumping losses.
Another way to improve fuel economy in a vehicle is to employ an engine as part of a hybrid electric or hybrid hydraulic propulsion system whereby an otherwise smaller displacement engine with improved fuel economy can be used and which allows the engine to be operated by automated control systems at a narrower range of engine speeds and outputs. Hanada (U.S. Pat. No. 6,886,524) discloses a variable displacement engine combined with a hybrid powertrain configuration. Although Hanada's system can offer improve fuel efficiencies, there are continuing efforts to further improve the efficiency of hybrid powertrains.